1. Field of the Invention
This invention relates to a method for producing a field-effect type semiconductor device, and more particularly, it relates to a method for producing a field-effect type semiconductor device, by which the production of transistor elements contained therein, involving control of a threshold voltage, can readily be conducted with high stability.
2. Description of the Prior Art
The recent amazing progress in the field of microelectronics centered with respect to integrated circuits have been bringing about a great social revolution comparable to the Industrial Revolution, together with such great advances as seen today in the field of optoelectronics and more particularly in the laser technology. Since the integrated circuit was first invented, the degree of integration in integrated circuits has become greater by leaps and bounds, and indeed there have been successively developed large scale ICs (LSIs), very large scale ICs (VLSIs), and ultra large scale ICs (ULSIs). Such high degrees of integration have been achieved through dimensional miniaturization of elements, improvement of device construction and circuits, and so on.
For example, in the production of field-effect type integrated circuits, together with improvement of the degree of integration, much effort is directed toward the miniaturization of transistor elements (particularly, the making of thinner gate insulator films). For thinner gate insulator films, however, some problems may be encountered in increasing a breakdown voltage and in controlling a threshold voltage of the device.
FIGS. 5A to 5D show a typical process for conventional fabrication of transistor elements used in field-effect type integrated circuits. First, in the surface region of a semiconductor substrate 101 such as those made of silicon, an oxide film 103 is formed by thermal oxidation, after which as shown in FIG. 5A, impurities such as boron ions are implanted in the semiconductor substrate 101 to form a P.sup.- -type region 102, thereby attaining control of a threshold voltage. Next, as shown in FIG. 5B, the oxide film 103 is removed by an etching technique, followed by formation of a gate insulator film 104 on the semiconductor substrate 101. Then, as shown in FIG. 5C, at the prescribed position on the gate insulator film 104, a gate electrode 105 is formed, which is made of a polycrystalline semiconductor material such as polycrystalline silicon. Thereafter, according to the producing process for conventional MOS type semiconductor devices, the N.sup.+ -type source/drain regions 110 are formed, interlayer insulating film 112 is deposited on the entire surface, and aluminum wirings 113 are formed, resulting in a transistor element of the field-effect type integrated circuits, as shown in FIG. 5D.
In such a method for producing field-effect type semiconductor devices, however, there will be some problems as follows:
(1) Because the gate insulator film 104 is formed after ion implantation conducted for the purpose of controlling a threshold voltage, crystal defects generated by the ion implantation in the semiconductor substrate 101 may be taken into the gate insulator film 104, thereby reducing the reliability of the device.
(2) Because the gate insulator film 104 is formed after ion implantation conducted for the purpose of controlling a threshold voltage, crystal defects generated by the ion implantation in the semiconductor substrate 101 may be redistributed within the semiconductor substrate 101 during the subsequent oxidation step, thereby degrading the degree of inversion voltage control. Furthermore, even if the dose of impurity ions is predetermined by considering the redistribution of impurity ions, the variation in time and temperature used in the formation of the gate insulator film 104 may be directly reflected in the scatter of inversion voltage values, thereby making it difficult to control the threshold voltage with high accuracy.
(3) Because the gate insulator film 104 is formed after ion implantation conducted for the purpose of controlling a threshold voltage, impurity ions generated by the ion implantation in the semiconductor substrate 101 may be diffused into the deep portion of the semiconductor substrate 101, thereby deteriorating the threshold voltage particularly of buried channel MOSFETs with shorter channel lengths. This imposes a restriction on the miniaturization of transistor elements contained in integrated circuits.
(4) In cases where the thickness of the gate insulator film 104 departs from the prescribed value, it is difficult to perform reoxidation for the purpose of adjusting the thickness of the gate insulator film 104 to the prescribed value.